Aluminothermic reactions are usually carried out in a conical crucible, in the narrowed lower region of which an outlet nozzle is exchangeably arranged. The outlet nozzle must be closed while the aluminothermic reaction is being carried out.
The molten metal which is formed during the aluminothermic reaction may only flow out of the outlet nozzle when the molten liquid slag, essentially comprising aluminum oxide, has separated from the molten metal and is floating thereon. If the outlet nozzle is tapped too early, there is a risk of incorrect casting owing to the reaction of the aluminothermic mixture being as yet incomplete or as a result of the separation between the aluminum oxide slag formed and the molten metal formed not yet being complete.
In order to increase the welding reliability, closures for the outlet nozzle are known which begin to melt and are melted through by the molten metal produced aluminothermically within a predetermined time.
For example, DE-C-32 11 831 describes a device for the automatic casting of aluminothermically produced steel, which device is composed of various small ceramic plates and an aluminum sleeve. This device is inserted into the outlet of the reaction crucible and serves as an automatically tapping closure, i.e. the closure melts through at a specific time and the outlet opens automatically without the intervention of a person, so that the liquid steel produced can flow out of the reaction crucible. This device is also known as an automatic tapping thimble.
Such an automatic tapping thimble usually comprises a Plug body, for example made from sand, a sleeve, for example made from aluminum, and one or more small closure plates, for example made from compressible fibrous aluminum silicate.
The material used to produce these closure disks has a melting point of .gtoreq.1650.degree. C. A preferred range indicated has a melting point of 1700.degree. C. .+-.20.degree. C.
WO 80/00546 describes a method for controlling the discharge from a tundish during the continuous casting of steel. In this method, first of all the outlet opening of the tundish is covered with a cover which consists of graphite, silicon or a combination of these elements.
This publication does not go into any detail with regard to the importance of the casting temperature or with regard to the temperature which is reached locally during the strongly exothermic thermite reaction, which can quite easily exceed the casting temperature.
It is even noted that penetration of the graphite by the steel is to be avoided.
The thermite reaction can be divided into two steps, the reduction reaction and the separation between Thermit steel and slag. Normally, each of these steps lasts on average about 10 seconds.
Then, the Thermit steel is available after approx. 10 seconds. This state corresponds to the tundish filled with steel during the continuous casting of steel. If, in the thermite reaction, the molten metal is discharged at the time described in WO 80/00546, i.e. only after up to 40 seconds after the liquid Thermit steel has become available, there is a risk of blocked tap, i.e. of the Thermit steel solidifying prematurely in the reaction crucible.
In order to achieve a homogeneous reaction product, it is necessary for the aluminothermic reaction to proceed as uniformly as possible and for this uniform reaction progress to be reproducible. If the reactions proceed at different rates, the alloy-forming ingredients may be burned off differently. This leads to alloys of different compositions and hence also to different properties.
If the reaction is carried out in a casting crucible, the bottom opening of which is sealed off by a thimble which can be melted through, as described, for example, in DE-C-32 11 831, it is intended that the thimble should be melted through within a precisely predetermined time interval after ignition of the mixture, in order to ensure that the reaction has ended and the slag has completely separated from the molten metal. If the thimble were to melt through too early, liquid slag particles which have not yet separated out could be entrained by the molten metal flowing out. If the thimble were to melt through too late, the melt may have already been cooled excessively and may therefore be in a state which is undesirable in particular during rail welding.
The cooling of the molten steel in the reaction crucible is combined with a cooling of the rail. The longer the tapping times, the more the rail cools down, thus making it more difficult to weld.
The residence time of the molten steel in the reaction crucible is therefore an important parameter for the welding of rails. The more uniform the opening times, also known as tapping times, of the automatic tapping thimble are, the more reproducible the welding conditions become, which simultaneously brings about a higher welding reliability.
Since ceramic fibers or other suitable fibrous materials which have a not inconsiderable compressibility are generally used in the automatic tapping thimbles according to the prior art, the tapping times are also affected to a considerable extent by the compression of these fibrous materials during the manufacture of the crucible plugs.
The use of the tapping thimbles of the prior art therefore leads to great fluctuations in the discharge times, which again has an adverse effect on the welding conditions and consequently also on the welding reliability.